The central nervous system (CNS) modulates muscle force by two mechanisms: through recruitment of motor units and through variation in their discharge rate. For the effective control of force, however, provision must be made by the CNS to account for the heterogenous properties of motor units comprising a muscle. The order in which a motor unit is recruited, as well as whether its force output is routinely modulated by changes in discharge rate, appears directly related to its physiological properties with respect to other motor units within the same muscle. At a higher level of organization, different control strategies must be played by the CNS to modulate force among muscles with diverse functions. Muscles with widely different functions often contain motor units with unique properties. This project is concerned with several key aspects of force modulation. Electrophysiological experiments will be carried out on selected muscles of the cat distal forelimb. Motor units supplying these muscles are known to differ in their mechanical properties and synaptic organization when compared to other areas of spinal motor organization. A long-term objective of this project is to determine what organizational principles are common between the forelimb and other areas of motor organization, and what specializations are unique to the forelimb because of the motor tasks it must perform. This, in turn, will provide valuable background information on how the segmental motor control system may be affected under a variety of neurological and injury-related disorders.